[Docs] Improve Convert Processor description (#35280)

Sometimes users are confused about whether they can use the Convert Processor
for changing an existing fields type to other types even if the existing one is already
ingested. This confusion is from the first line of description. Changing this and also
adding a some detail to the code snippet.

docs/reference/ingest/ingest-node.asciidoc

@@ -828,7 +828,7 @@ include::ingest-node-common-processor.asciidoc[]
 
 [[convert-processor]]
 === Convert Processor
-Converts an existing field's value to a different type, such as converting a string to an integer.
+Converts a field in the currently ingested document to a different type, such as converting a string to an integer.
 If the field value is an array, all members will be converted.
 
 The supported types include: `integer`, `long`, `float`, `double`, `string`, `boolean`, and `auto`.
@@ -857,11 +857,17 @@ include::ingest-node-common-processor.asciidoc[]
 
 [source,js]
 --------------------------------------------------
+PUT _ingest/pipeline/my-pipeline-id
 {
-  "convert": {
-    "field" : "url.port",
-    "type": "integer"
-  }
+  "description": "converts the content of the id field to an integer",
+  "processors" : [
+    {
+      "convert" : {
+        "field" : "id",
+        "type": "integer"
+      }
+    }
+  ]
 }
 --------------------------------------------------
 // NOTCONSOLE